> I don't know what a "Valid HTML 2 document" is, but documents conforming
> to RFC 1866 (HTML 2.0) must start with a document type declaration.


Quoting said document:

      NOTE - If the body of a `text/html' message entity does not begin
      with a document type declaration, an HTML user agent should infer
      the above document type declaration.

This is what makes documents without a DOCTYPE HTML 2.0.  An HTML 4.01
document without such a line should be parsed as HTML 2.0 and fail, but
an HTML 2.0 document will succeed.

At one time validator.w3.org would issue valid HTML 2.0 stickers to
documents with no DOCTYPE but otherwise compliant.  I think the catalog
contained a line that implemented the default.